A trade skill is a skill that uses crafting to make money.
These skills are equally available to each class. Some skills come in pairs, one to collect raw materials and one to turn them into more valuable items before selling them:
Instead of cooking fish, you can also use Cooking with items dropped by certain creatures, such as Raw Steaks and Gaper Stalks. To kill those creatures and obtain the items, you need a combat skill.
Other trade skills include:
However, Alchemy is only available to clerics and druids. Alchemy is another source of money, if you choose to sell or otherwise trade the potions instead of consuming them.
So too for the Wizard Enchanting, which is another crafting Prime, relying on harvesting logs and /or ferrite(s) to extract Etherite, from, also used in Enchanting.